1. Field of the Invention
The present invention relates to an image data encoding technique.
2. Description of the Related Art
Recent spread of personal computers and mobile terminals has led to broad use of digital data communication (data communication) via the Internet. Digital data used in data communication include images. As a characteristic feature of digital images, for example, copy, search, and various kinds of image processing are relatively easy. Hence, a lot of people handle digital images.
For efficient use of digital images, image compression to reduce the file size is indispensable. Typical techniques are JPEG and PNG. Image compression may distort an image. Techniques of suppressing image quality degradation have been researched and developed. Techniques of attaining not only the primary function of reducing the file size but also attaining various other functions have also been developed. For example, JPEG 2000 enables encoded data generation by scalably decoding image data gradually from a low resolution up to its original resolution.
Techniques having an image quality improving function in addition to the above-described resolution scaling function have also been developed. For example, Japanese Patent Laid-Open No. 2006-197573 discloses this technique. This reference or a technique based on this reference is called HD Photo® in general. More specifically, an image is segmented into tiles. DCT (Discrete Cosine Transform) is repeatedly performed for the respective tiles, thereby generating stream data of layers. To suppress block distortion generated among the tiles, predetermined processing is executed using the pixels of two adjacent tiles before or during the DCT.
The above reference aims at improving the quality of a decoded image having the same resolution as the input image.
Recent image sensing apparatuses represented by a digital camera have a resolution of more than 10,000,000 pixels. The resolution of an original sensed image is too high for an information processing apparatus represented by a computer to display, so the apparatus can display only part of it. It is therefore necessary to generate and display image data at a resolution lower than that of the original image. In the above-described reference, however, if an image having a resolution lower than the original resolution is generated, the image quality may degrade because the technique only aims at improving the quality of a decoded image having the same resolution as the original image.